Functional component:

const handleFocus = (event) => event.target.select();
const Input = (props) => <input type="text" value="Some something" onFocus={handleFocus} />

ES6 class component:

class Input extends React.Component {
    handleFocus = (event) => event.target.select();

    render() {
        return (
            <input type="text" value="Some something" onFocus={this.handleFocus} />
        );
    }
}

React.createClass:

React.createClass({
    handleFocus: function(event) {
      event.target.select();
    },

    render: function() {
      return (
        <input type="text" value="Some something" onFocus={this.handleFocus} />
      );
    },
})

Another Way Functional Component with useRefHook:

const inputEl = useRef(null);

function handleFocus() {
  inputEl.current.select();
}

<input
  type="number"
  value={quantity}
  ref={inputEl}
  onChange={e => setQuantityHandler(e.target.value)}
  onFocus={handleFocus}
/>

let's add the simplest based on @dschu's answer:

...
<input type='text' value='Some something' onFocus={e => e.target.select()} />
...

var React = require('react');

var Select = React.createClass({
    handleFocus: function(event) {
        event.target.select()
    },
    render: function() {
        <input type="text" onFocus={this.handleFocus} value={'all of this stuff'} />
    }
});

module.exports = Select;

Auto select all content in a input for a react class. The onFocus attribute on a input tag will call a function. OnFocus function has a parameter called event generated automatically. Like show above event.target.select() will set all the content of a input tag.